1. Field of the Invention
The invention relates to a general purpose apparatus for digitizing an analog signal, and more particularly to a general purpose apparatus for digitizing an analog signal having an inherent noise component.
2. Description of the Related Art
A programmable optical analyzer is an instrument for detecting and analyzing light. The analyzer can be adapted to a wide variety of applications by means of software. One class of analyzers employs a charged coupled device (CCD) as an optical sensor and an analog-to digital convertor to convert sensed data into a number.
Programmable CCD read rates make such an analyzer general purpose. By providing a relatively fast read rate, a medium read rate, and a relatively slow read rate, the analyzer can be used for different types of applications. Certain scientific applications may require a relatively high quality signal at the relatively slow read rate. In contrast, image applications may require the relatively fast read rate, but may be able to tolerate signal quality degradation resulting from a fast read rate.
Programmable temperature control also makes the analyzer general purpose, because the optimal operating temperature of the CCD may vary from application to application. For some applications, it is desirable to cool the CCD to as low a temperature as practical in order to limit the amount of dark current generated in the CCD. CCD sensor performance anomalies can occur at very low temperatures, however, making these very low temperatures unsuitable for other types of applications. For example, in spectroscopic applications there is typically a sharply focused line of light imaged onto the surface of the CCD. At very low temperatures one side of the line will be smeared during the data readout. Further, at very low temperatures CCD sensors become less sensitive to the red end of the spectrum making the very low temperatures unsuitable for other types of applications.
Programmable exposure time control allows the analyzer to be used in both high and low light experiments. Exposure time is typically controlled with a shutter.
In summary, a general purpose programmable optical analyzer allows users to trade off certain performance specifications in order to enhance other performance specifications. In designing such a general purpose analyzer, design decisions that may preclude a class of applications should be avoided. For example, although some applications may be concerned with only a portion of the dynamic range, a general purpose analyzer that optimizes resolution at one end of the dynamic range at the expense of resolution at another end of the dynamic range would be undesirable, because such a trade-off might preclude applications that are concerned with resolution at the other end of the dynamic range.